Droplet-based capacitive pressure sensor

ABSTRACT

A pressure sensing apparatus which utilizes an electrolytic droplet retained between a first and second sensing electrode within a housing. Contact between the electrolyte droplet and the electrodes form electric double layers (EDL) having interfacial EDL capacitance proportional to interface contact area which varies in response to mechanical pressure applied to deform exterior portions of the housing. The electrolyte contains a sufficient percentage of glycerol to prevent evaporative effects. Preferably, the sensing electrodes are modified with depressions, hydrophilic and/or hydrophobic portions to increase central anchoring of the electrolyte droplet within the housing. The inventive pressure sensor provides high sensitivity and resolution which is beneficial to numerous applications, and is particularly well-suited for medical sensing applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional patent applicationSer. No. 61/737,712 filed on Dec. 14, 2012, incorporated herein byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Grant Nos.ECCS-0846502 and EFRI-0937997 awarded by the National ScienceFoundation. The Government has certain rights in this invention.

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED IN A COMPUTER PROGRAMAPPENDIX

Not Applicable

NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

A portion of the material in this patent document is subject tocopyright protection under the copyright laws of the United States andof other countries. The owner of the copyright rights has no objectionto the facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the United States Patent andTrademark Office publicly available file or records, but otherwisereserves all copyright rights whatsoever. The copyright owner does nothereby waive any of its rights to have this patent document maintainedin secrecy, including without limitation its rights pursuant to 37C.F.R. §1.14.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains generally to pressure sensing devices, and moreparticularly to a droplet-based capacitive pressure sensing device.

2. Description of Related Art

Microfluidic-based sensors have been an active area of research fortheir excellent flexibility, high sensitivity, simple fabrication, andwide adaptability. A variety of sensing and actuation mechanisms havebeen incorporated in the development of microfluidic sensing devices,the majority of which rely on sensing changes in a physical property(e.g., optical, electrical or mechanical) induced by fluidicdisplacement, and/or new material functionality introduced to workingfluids (e.g., as optical and electromagnetic waveguides).

However, the existing microfluidic sensors suffer from one or moreshortcomings, such as being influenced by environmental effects, and/orinsufficient pressure sensitivity and resolution.

Accordingly, the present invention provides ultrahigh levels of pressuresensitivity and resolution, while overcoming numerous environmentalsensitivity issues of prior microfluidic sensors.

BRIEF SUMMARY OF THE INVENTION

A droplet-based capacitive pressure sensing device is described whichutilizes an elastic electrolyte-electrode contact with large interfacialcapacitance that achieves ultrahigh sensitivity and resolution (e.g.,1.58 μF/kPa and 1.8 Pa, respectively). In addition, the inventivepressure sensor is simple to fabricate, has mechanical flexibility,optical transparency, while being insensitive to both evaporation andthermal noise.

Further aspects of the invention will be brought out in the followingportions of the specification, wherein the detailed description is forthe purpose of fully disclosing preferred embodiments of the inventionwithout placing limitations thereon.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 is a top view of a droplet-based capacitive pressure sensoraccording to an embodiment of the present invention.

FIG. 2 is a cross-section view of the droplet-based capacitive pressuresensor seen in FIG. 1 according to an embodiment of the presentinvention.

FIG. 3 is a schematic of an equivalent circuit of the droplet-basedcapacitive pressure sensor according to an embodiment of the presentinvention.

FIG. 4 is an image rendition of contact angles for variousIndium-tin-oxide (ITO) surfaces utilized according to an embodiment ofthe present invention.

FIG. 5 is a graph of capacitance with respect to frequency for thevarious ITO surfaces depicted in FIG. 4.

FIG. 6A through FIG. 6D are graphs of capacitance with respect topressure for various pressure sensing geometrical characteristicsutilized according to one or more embodiments of the present invention.

FIG. 7A through FIG. 7C are graphs of response and environmentalinfluence comparing different droplets utilized according to anembodiment of the present invention.

FIG. 8 is a graph of blood pressure variation determined with thedroplet-based capacitive pressure sensor according to an embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

1. Introduction.

The inventive droplet-based capacitive pressure sensor mechanism isdescribed utilizing a highly capacitive electric double layer (EDL)presented at an extremely flexible droplet-electrode interface. Inparticular, the device may be implemented on a simple suspended membranestructure, the EDL offers high charge density at the nanoscopicionic-electronic interface to establish an ultrahigh interfacialcapacitance, while the elastic deformability of electrolyte droplets onhydrophobic-modified electrodes allows reversible fluidicexpansion/contraction, in response to external mechanical stimuli (i.e.,pressure). The interfacial droplet sensor achieves an ultrahigh pressuresensitivity of 1.58 μF/kPa along with ultrahigh resolution of 1.8 Pa,which is comparable to the highest values reported in the literature. Inaddition, the response time of the sensing device (e.g., 260 ms) hasbeen characterized under a constant membrane deformation (e.g., of 50μm). The present invention also illustrates that through changes of thefluidic medium (e.g., lower viscosity) or modified surface energy (e.g.,increased hydrophobicity), the mechanical response can be improvedsignificantly.

The droplet sensing device is comprised of two flexible polymermembranes with conductive coating and a separation layer with a sensingchamber hosting the electrolyte droplet, which can be opticallytransparent and mechanically flexible. Moreover, adding glycerol to ahighly conductive electrolyte droplet, such as at least 25% or more,addresses the primary evaporative concern with long-term stability forsuch a liquid-based sensor under room condition (46% humidity and 24°C.). Theoretical analyses and experimental investigations on key designparameters (i.e., the radius and height of the sensing chamber anddroplet size) have been thoroughly conducted to characterize andoptimize the overall device performance. Furthermore, the performance ofthe droplet sensors under different temperatures and humidity levelsupon reaching thermodynamic equilibrium has been investigated. As aproof of applicability, the droplet-based capacitive pressure sensor wassuccessfully applied to detect minute blood pressure variations on theskin surface (with the maximum value less than 100 Pa) throughoutcardiovascular cycles.

2. Operating Principles.

The electric double layer (EDL) provides a large capacitance per unitarea at the nanoscopic interface between an electrode and electrolyte(as high as tens of μF/cm²). Several theoretic models have been proposedto explain the high levels of interfacial capacitance, established bymobile electrons in a conductive solid phase and counter-ionsimmigrating in the adjacent liquid environment. The interfacialcapacitance is influenced by the surface charge density and Debyelength, which is used to describe the thickness of the EDL whenelectrostatic and thermodynamic activities reach equilibrium in thesolution phase. Surface charge density is influenced by thephysiochemical property of the interface, kinetic energy of the ionicspecies, electric potential as well as permittivity and concentration ofthe solution.

FIG. 1 and FIG. 2 illustrate an example embodiment 10 of the inventivedroplet-based capacitive pressure sensor which utilizes droplet-basedinterfacial sensing that achieves ultra-large interfacial EDLcapacitance at the highly elastic droplet-electrode contact. Theinterfacial EDL capacitance of the sensor is proportional to the area ofthe contact interface, unlike the solid-state strain gauges, whichmeasure the change of bulk resistance under mechanical deformation.

As can be seen in the top and cross-section views of FIG. 1 and FIG. 2,respectively, the sensor is bounded on top 12 a, and bottom 12 b with asubstrate (e.g., Polyethylene Terephthalate (PET), orPolydimethylsiloxane (PDMS)) whose inner surfaces 14 a, 14 b areconductively coated (e.g., Indium-tin-oxide (ITO)) to facilitate anultra-large interfacial capacitance in relation to the retainedelectrolytic fluid droplet 20 (e.g., glycerol electrolyte). It should beappreciated that at least one of the layers (top or bottom) along withits conductive coating must be flexible, that is to say deformable,whereby applied pressure can deform the membrane and the retaineddroplet. Sidewalls 16 (e.g., Polydimethylsiloxane (PDMS)) are interposedbetween the inner surfaces 14 a, 14 b to complete encapsulation of thepressure sensor leaving gaps 18 (e.g., air or other gaseous dielectric)between fluid droplet 20 and sidewalls 16. In the example embodiment,the two layer structure is shown in a circular plan form, although itshould be appreciated that other shapes (e.g., any bounded geometricshapes or combination of shapes, including square, rectangular, otherpolygons, oblong, freeform, etc.), can be utilized to retain the dropletbetween the upper and lower surfaces within its surrounding. Contacts 22a, 22 b are seen in FIG. 1 exemplified as conductive tabs extending fromdevice 10, while in FIG. 2 the contacts are shown schematically inrelation to sensing voltage 24 V_(s).

In considering FIG. 2, the principle of droplet-based interfacialcapacitive sensing can be more readily discerned. Sensing chamber 18within the elastic separation layer hosts an electrolyte droplet 20,which is sandwiched between the two membranes (e.g., polymeric) 12 a, 12b with inner surfaces coated with conductive material 14 a, 14 b, whichis preferably transparent. An electric double layer (EDL) formsimmediately upon the droplet-electrode contact (i.e., 14 a-20-14 b),with mobile electrons migrated from the conductive membrane surface anda counter-ion layer accumulated from the electrolyte solution inresponse to application of sensing voltage V_(s) as seen in the figure.

Ventilation channels 26 (vent) are shown in FIG. 1 and FIG. 2 extendingthrough portions of sidewalls 16, into gap 18 between the sidewall anddroplet, which provides pressure equalization in response to deflectionof the membrane. It should be appreciated that these vents 26 can beimplemented in a number of different ways without departing from theteachings of the present invention. Alternatives include a plurality ofsmaller vents in sidewalls 16 (e.g., laser cut), or a sufficientlyporous material in constructing sidewalls 16, and so forth. In addition,the vents can be configured in the upper and/or lower membranes. Thepreferred type and location for the vents can depend on both thecomposition of the electrolyte droplet and the application to which thesensor is being utilized.

Under the application of an external mechanical load, the suspendedpolymer membranes and the separation layer deforms elastically, and as aresult, the contact area of the droplet-electrode interface experiencescircumferential expansion (assuming incompressible fluid with unalteredvolume of the droplet). Given a relatively constant charge density, thevariation in the contact area results in a proportional change in theinterfacial capacitance.

FIG. 3 illustrates an equivalent circuit 10 for the sensing structureshown in FIG. 1 and FIG. 2. In this equivalent circuit, the EDLcapacitances (C_(EDL)) at the droplet-electrode interfaces are modeledas two capacitors connected through a resistive element (R_(bulk))expressing the bulk resistance of the conductive droplet, and showingsensing voltage contacts V_(s). The overall deformation of the sensingchamber, which includes the deflection of the flexible membrane and thecompression of the elastic separation layer, leads to a change ofinterfacial contact area, and therefore changes the capacitance of thedevice.

More specifically, small deflections of these membranes can bemathematically predicted according to the classic thin-plate theory,while elastic deformation of the separation layer is well adapted to thelinear strain-stress relationship. As aforementioned, the unit areacapacitance (c_(o)) of the EDL can be considered as an experimentallydetermined constant. Therefore, the overall interfacial EDL capacitance(C_(EDL)) can be simply calculated from the product of the unit areacapacitance and the droplet-electrode contact area. Themechanical-to-electrical sensitivity of the sensing device can thus beanalytically expressed as:

$\begin{matrix}{\frac{\Delta\; C_{EDL}}{P} = {{c_{o}( {{\alpha\; R^{2}} + {\beta\; H}} )}\frac{V_{d}R^{2}}{H^{2}}}} & (1)\end{matrix}$where P is mechanical load, α and β represent membrane deflection andelastic deformation of the separation layer, respectively, and can bedetermined by the geometrical and mechanical properties of the sensingmembrane and the separation layer. The values R and H represent radiusand height of the sensing chamber, respectively, while V_(d) indicatesthe volume of the electrolyte droplet. As can be seen, both the membranedeflection (1st term) and separation layer deformation (2nd term)contribute to the overall mechanical-to-capacitive sensitivity. Itshould be recognized that both the hydraulic pressure of the droplet andthe radius of the curvature at the droplet-electrode interface have beenignored in the simplified mathematical expression. In addition,gravitational effects have been ignored, since the droplet volume istypically confined to be smaller than the cube of the capillary length,within which the capillary force is dominant. It should be appreciatedthat within the capillary length, the capillary force is dominant, whilebeyond the capillary length, gravity becomes more important.

3. Materials and Methods.

3.1 Surface Modification.

In at least one example embodiment of the device, indium-tin-oxide(ITO)-coated (100 nm thick) polyethylene terephthalate (PET) films (125μm thick) were utilized to serve as pressure sensing membranes as wellas to establish the interfacial droplet-electrode contact in the device.The ITO coated substrate exhibited excellent optical clarity, with atransmission coefficient of 80.9% at visible wavelengths, a highelectrical conductance (sheet resistance of 50Ω/□ (ohms/square)), andstrong mechanical properties (Young's modulus of 3-4 GPa).

In order to reduce the hysteresis of droplet deformation, simple surfacemodification was introduced to the conductive coating. In brief,ITO-coated PET sheets were exposed to oxygen plasma at 30 watts (W) for30 seconds to induce surface hydroxylation. Following plasma activation,a PDMS stamp, made from a mixture of a base and a curing agent at 10:1weight ratio (SYLGARD 184® from Dow Corning®) and thermally cured at 80°C. for 2 hours was brought into physical contact with the conductivecoating enriched with hydroxyl groups for 2 hours, during which ananometer-thick layer of PDMS oligomers was transferred and immobilizedonto the electrode surface. In addition, an indented hole (e.g., of 200μm in diameter) was included in the center of the PDMS stamp, leaving ahydrophilic spot surrounded by the hydrophobic oligomer layer on theelectrode surface, by which the sensing droplet is preferably anchoredand stabilized in the middle of the sensing chamber. For the sake ofenhancement of the surface hydrophobicity and thereby reduction of thedroplet response time, a superhydrophobic nanocomposite material (e.g.,PFC M1604V, FluoroPel™) was spray-coated onto the PDMS-oligomer-modifiedITO surface. The commercial superhydrophobic coating was measured withwater contact angle (CA) of 155°.

3.2 Device Fabrication.

The inventive droplet-based capacitive pressure sensors can be readilyfabricated in a number of alternative ways. In one embodiment, thedevices were fabricated utilizing direct laser micromachining whichprovided a facile approach to form the geometrical shape of the sensingmembranes as well as the PDMS separation layer containing a sensingchamber in one single step. Specifically, a desktop CO₂ pulsed laserengraver can be utilized with a graphic user interface (e.g., CorelDrawor Photoshop) to perform the laser-etching process. Various powerintensities from 0.3 W to 3 W were applied to trim ITO-coated PET orPDMS substrates of different thicknesses, from which a minimal featureresolution of 100 μm was reliably achieved. One or more ventilationchannels were formed in the sensor chamber, being preferably engravedthrough the PDMS separation layer to maintain pneumatic pressure balanceduring chamber deformation. In a subsequent step, the laser-trimmed PETsubstrates with ITO coating were imprinted with PDMS oligomers aspreviously described and bonded to the PDMS separation layer throughoxygen plasma-assisted bonding (e.g., at 90 W for 30 seconds). Prior tofinal assembly of the device, an electrolyte droplet with a desiredvolume was dispensed by a micropipette in the center of the sensingchamber.

3.3 Electrical and Mechanical Characterization.

Interfacial capacitance of the EDL layer can be assessed electrically,such as by utilizing a capacitance meter or multi-meter (e.g.,inductance-capacitance-resistance (LCR) meter). At the AC excitationvoltage of 0.5V applied to V_(s) seen in FIG. 2 and FIG. 3, theinterfacial capacitor was connected to the LCR meter in a bipolarconfiguration, sweeping from 20 Hz to 100 kHz for acquiring sub-MHzresponses of the EDL layer. A high-speed digital camera (e.g., up to1,200 frames per second) was used to capture the shape change of thedroplet in order to measure droplet transient mechanical response.External mechanical point loads were applied onto the center of thesensing membrane through a custom-built motorized force gauge with 1 mNresolution, driven by a computer-controlled step motor with asufficiently fine spatial resolution (e.g., 0.2 μm). Themechanical-to-capacitance responses were evaluated twice on twoidentical devices for each parameter. For the resolution measurement,minute droplets (e.g., 50 μL volume) dispensed by a micropipette wereapplied directly onto the sensing membrane until a noticeable capacitivechange appeared in the LCR meter, which was evaluated three times forthe same device.

4. Results.

4.1 Sensing Droplets.

In order to achieve the proper interfacial sensing, the physicalproperties of the sensing droplet should satisfy the following criteria:(a) high ionic concentration to ensure high electrical conductance andinterfacial capacitance, (b) polarized molecular structure whereinhydrophobic surfaces have reversible elasticity, and (c) low fluidicviscosity which supports rapid mechanical response. Aqueous-basedelectrolyte solution (e.g., NaCl) with high ionic concentration are asimple choice, satisfying all criterion except for exhibiting moderateevaporation under normal room air conditions of temperature, pressureand humidity. Mixing an aqueous solution with an anti-evaporative agent,such as glycerol, effectively reduces evaporation due to decreased vaporpressure. However, glycerol is electrically non-conductive and hassubstantially higher viscosity than that of water (about 1,400 timesgreater). Therefore, an optimal mixing ratio of glycerol and electrolytesolution was sought in the inventive implementations.

Table 1 summarizes the physical and chemical properties of theelectrolyte/glycerol mixture (given a NaCl solution at the concentrationof 1.1 mol/L) at various mixing ratios (v/v %), which includeselectrical conductivity, unit-area capacitance (c_(o))) at 20 Hz,evaporation, viscosity (μ), relaxation time (t) and contact angle (θ)(all measured on PDMS oligomer-coated ITO surfaces). As can be seen inthe table, the mixing ratio has a minimal influence on electricalconductivity and unit-area capacitance, except for pure glycerol.Importantly, the 25/75% mixture of electrolyte/glycerol presents noappreciable evaporation under a regular laboratory environment (46%humidity and 24° C.) after 24 hours, as compared to a pure glycerolsample, which becomes hygroscopic under the same condition. Moreover,adding more glycerol to the mixture drastically increases the dynamicviscosity from 1.0 to 1412 Pa·s, and the 25/75% mixture becomes 60 timesmore viscous than that of water, which considerably affects therelaxation time, shown in the table increasing 0.02 seconds to 0.28seconds). In addition, glycerol and water have similar contact angles onthe hydrophobic coating (i.e., PDMS oligomer-coated ITO surface), and asa consequence, various mixtures have similar contact angles presented onthe hydrophobic surface.

In summary, considering the physical properties and evaporativestability of the mixtures, a mixture including at least 25% glycerol inthe electrolyte of the droplet is preferred. More preferably, a 25/75%electrolyte/glycerol solution was found to be a preferred working fluidfor the sensing droplet.

4.2 Surface Modification.

As described in a previous section, in at least one embodiment surface,hydrophobic treatment was applied to the conductive coating, towardensuring reversible and elastic deformability of the sensing interface.As previously reported, an ultrathin layer of PDMS oligomers can beuniversally transferred and immobilized on a hydroxylated surface. Theinterfacial oligomer layer of approximate nanometer thickness (e.g.,around 1-2 nm) does not significantly alter EDL uponelectrolyte-electrode contact. The added oligomer layer beneficiallypresents substantially reduced interfacial energy.

FIG. 4 depicts the variation in surface energy during the oligomertransfer process, by measuring the contact angle of a mixedelectrolyte/glycerol solution (25/75%). In the far left of this figure adroplet is seen on the ITO coating with a contact angle of 66°. With anoxygen-plasma treatment, the ITO-OH coating renders apparenthydrophilicity to the surface and contact angle reduces from 66° to 15°.Following the oligomer transfer step, the PDMS oligomer-coated ITO-PDMSoligomer surface switches the polarity from hydrophilicity tohydrophobicity with a contact angle of 90° being achieved, which furtherenhances the elasticity and reversibility of the droplet-electrodecontact by reducing adhesive energy of the liquid to the substrate. Acontrol is seen at the far right of pure electrolyte solution on theoligomer coated ITO surface.

4.3 Interfacial Capacitance.

The EDL capacitance, established by mobile electrons at the conductivesolid surface and a counter-ion layer accumulated in the electrolytesolution, is frequency-dependent in nature with several mechanismsassociated (e.g., electrophoresis and interfacial polarization).

FIG. 5 compares the frequency responses of the electrolyte mixture onthe original ITO surfaces in relation to the modification seen in FIG.4. As expected, the surface modifications have marginal influence on theinterfacial EDL capacitance, and it is most likely that neither surfacehydroxylation nor oligomer transfer alters the physical separationbetween the electrolyte and electrode interface. Moreover, in a controlexperiment, a pure electrolyte solution (given NaCl at the concentrationof 1.1 mol/L) exhibits a similar frequency response to that of theelectrolyte mixture on the PDMS oligomer modified surface within thesub-MHz range, which reconfirms that the mixture maintains sufficientelectrical properties at the interface. In this study, it was alsodetermined that the measured unit area capacitance was consistentlylower than previously reported, which is possibly due to the molecularstructure of the ITO layer prepared by different coating techniques.

4.4 Mechanical-to-Capacitive Sensitivity.

As presented in the section on operating principles, the overallmechanical-to-electrical sensitivity (ΔC_(EDL)/P) can be determined bythe geometrical confinements (the radius R and height H of the sensingchamber and the thickness of the polymeric membrane), droplet volume(V_(d)), and the material properties of the construct (Young's modulusand Poisson's ratio), given a fixed unit-area capacitance (c_(o)). Amongthose parameters, the radius of the sensing chamber generally comprisesthe more important role (4th power in membrane deflection) as the theorypredicted, followed by the height of the chamber (inverse 2nd power inmembrane deflection). In addition, the system sensitivity was found tobe directly proportional to the volume of the sensing droplet.Experimental investigations have been conducted to verify the abovetheoretical predictions.

4.5 Influence of Chamber Radius.

FIG. 6A and FIG. 6B depict capacitive changes over a wide spectrum ofpressure loaded on the droplet sensors with different sensing chambersizing, whose radius was varied from 1.5 to 9.0 mm, given a chamberheight of 200 μm and the droplet volume of 0.3 μL. The experimentalmeasurements (dots) were plotted in comparison with the valuescalculated from Eq. 1 (curves), and the slope rate of each devicemeasurement defines the corresponding device sensitivity. As predictedby the sensing theory, the radius exhibits 4th power in membranedeflection and 2nd power in the elastic deformation, which has asignificant effect on the system sensitivity. Within the smalldeflection limit, the capacitive charges change linearly with theexternal load as one would expect. In the devices with the largestsensing chamber (of 9.0 mm radius), the highest sensitivity of 90.2nF/kPa was achieved, as seen in FIG. 6A. In comparison, as the radius isreduced to two thirds of its former value (6.0 mm), the systemsensitivity drops drastically to less than 20% of that (17.2 nF/kPa)which closely correlates with our theoretical prediction, i.e., thesensitivity is directly proportional to the 4th power of radius, as themembrane deflection dominates the overall mechanical deformation. Theabove data suggests that the theoretical model fits the experimentsreasonably well under the assumptions. In both cases, the contact areabetween the droplet and the deformed membrane is smaller than 5% of thewhole chamber area, and therefore can be approximated as a flatinterface instead of curved. However, in the smallest devices (with aradius of 1.5 mm), the measurements deviate considerably (more than 40%)from the stimulated values. In this case it is highly possible that thesmall deformation limit, which is associated with the ratio of themaximum deflection to the radius of the membrane, has been exceeded. Inaddition, it has been observed that the radius of the curvature of thedeflected membrane can no longer be ignored in the smallest sensingunit. Overall, the radius of the chamber is a determinant factor for theoverall mechanical-to-electrical sensitivity, given its 4th powerrelation, as the membrane deflection serves as the primary mechanicaldeformation mechanism under external load.

4.6 Influence of Chamber Height.

As predicted by the theoretical analysis, the device sensitivity isinversely proportional to the 2nd power of the height of the sensing asthe membrane deflection dominates, and thus, a lower sensing chamberwill lead to a larger contact area at a given droplet volume.

FIG. 6C depicts sensitivity variations over four different chamberheights (140 μm, 210 μm, 280 μm and 350 μm) provided with a chamberradius of 6.0 mm and droplet volume of 3 μL. As can be seen, theshortest chamber (140 μm in height) shows the highest sensitivity (374nF/kPa) due to the presence of the largest contact area at thedroplet-electrode interface. As the height of the chamber is increasedfrom 140 μm to 210 μm, the device sensitivity decreases from 374 nF/kPato 144 nF/kPa, closely following the negative quadratic relationshipbetween the sensitivity and the chamber height. The tallest chamber of350 μm exhibits the lowest level of sensitivity at 54.5 nF/kPa.Moreover, as chamber height increases, the separation layer becomesincreasingly influential on the overall sensitivity. In another words,the thicker separation layer produces more substantial deformation underthe same load according to the linear strain-stress relationship.However, in the exemplified range of height, the maximal deformation ofthe separation layer is still one order of magnitude smaller than thatof the membrane deflection as expected. In brief, all four sets of thesensitivity measurements are in close agreement with the theoreticalanalysis presented above.

4.7 Influence of Droplet Volume.

It should also be appreciated that droplet volume is directlyproportional to system sensitivity. In general, a larger droplet coversmore interfacial area with a longer circumferential periphery along thecontact, which leads to a higher sensitivity given constant dimensionsof a sensing chamber. However, the increased droplet size can also leadto non-linear response as the droplet-electrode interface can no longerbe approximated as a planar surface.

FIG. 6D depicts four different droplet sensing volumes of 0.3 μL, 0.6μL, 1.2 μL and, 3.6 μL, respectively, characterized and compared underidentical sensing chamber dimensions (6.0 mm radius and 200 μm height).It can be seen from the figure, that as droplet volume increased from0.3 μL to 0.6 μL, and then to 1.2 μL, the corresponding sensitivity rosein a linearly proportional manner from 17.2 nF/kPa to 32.1 nF/kPa, andthen to 65.7 nF/kPa. The highest sensitivity (218 nF/kPa) was achievedon the sensing droplet of 3.6 μL in volume. These findings re-confirmthe applicability of Eq. 1.

4.8 Optimization of Device Sensitivity.

An embodiment of the present invention was optimized in response to theabove observations. Accordingly, the device optimized for sensitivity is9.0 mm in radius, 140 μm in height and 3 μL in droplet volume whichprovided sensitivity of 1.58 μF/kPa, which closely compares with thehighest value we found reported in the literature, which was 2.24 μF/kPaand which relied upon a mercury droplet in combination with an ultrahighpermittivity material. In addition to the design parameters discussedabove, the thickness of the polymeric membrane is another crucialparameter in determining sensing performance. Sensitivity caused bymembrane deformation is inversely proportional to the cubic power of itsthickness. In this study, we have been using the off-the-shelfITO-coated substrate (125 μm in thickness); however, further reducingthe membrane thickness could lead to drastic increases in devicesensitivity. Moreover, in the optimal sensing design, the device showsan extremely high pressure resolution of 1.8±0.4 Pa.

4.9 Response Time.

Response time serves as another critical measure for mechanical sensingdevices in addition to their sensitivity and resolution. In general, theresponse time of such a droplet-based sensor can be influenced by theviscosity of the droplet medium, the hydrophobicity of the electrodesurface, and the elasticity of the PDMS separation layer and the PETmembrane, of which the liquid viscosity and surface hydrophobicitydominate. Accordingly, we have further characterized the response timeof the geometrically identical sensing devices with varied dropletviscosity and surface hydrophobicity under the same level of themembrane deformation of 50 μm.

FIG. 7A depicts response times for different droplet systems, comparinga control (far left) with an electrolyte only droplet (center), and adroplet with surface treatment (far right). As can be seen, the responsetime is around 260 ms for a control droplet in an optimized sensingdesign having a chamber of 6 mm in radius and 140 μm in height and thevolume of the droplet is 3 μL. With decreased droplet viscosity (from 60Pa·s of the 25/75% electrolyte/glycerol mixture to 1 Pa·s of a pureelectrolyte solution) and the enhanced surface hydrophobicity (from CAof 90° to 155°) can lead to more than two-fold reduction in the responsetime (to around 100 ms). However, the pure electrolyte solution issubject to evaporation effects, while converting the electrolyte surfacefrom hydrophobic to superhydrophobic introduces a micrometer-thick(around 10 μm) dielectric layer, which substantially lowers interfacialcapacitance.

4.10 Influences of Temperature and Humidity.

Importantly, the performance of the droplet sensors can be highlysubject to environmental humidity level. Specifically, theelectrolyte/glycerol mixture establishes thermodynamic equilibrium withdifferent stable mixing ratios at different humidity levels by eitherlosing (evaporation) or gaining (condensation) aqueous contents to/fromthe environment.

FIG. 7B depicts sensitivity in relation to changing humidity levels,seen in a control at room conditions having humidity of 46%, and athumidity levels of 50%, 60%, 70% and 80%. It will be appreciated thatthe volume and capacitance of a droplet sensor can substantially deviatefrom the initial condition (of 25/75% electrolyte/glycerol) as themixture compositions changes. In other words, as the humidity levelrises, the volume of the droplet would likely increase as additionalmoisture condensates into the mixture. However, once a new thermodynamicequilibrium is established, the capacitance will remain stable.Therefore, in practice one can either employ sensing elements withdifferent mixing ratios at different humidity levels, or allow thesensing element to equilibrate with the environment thermodynamicallyprior to its usage.

In contrast, the environmental temperature fluctuation has only posedminor influence on the interfacial capacitance, in comparison with theresistance-based devices.

FIG. 7C depicts electrolyte-electrode interfacial capacitance measuredat three separate temperatures of 4° C. (reduced), 25° C. (regular), and45° C. (elevated), respectively, from which less than 10% variation inthe unit-area capacitance has been observed at the steady state. Incomparison, within the same range of temperature fluctuation, theresistance measurement can fluctuate up to 55% in what we've seen onpressure sensing devices reported in the literature.

5. Demonstration of Example Application.

To demonstrate the utility of the simply constructed and mechanicallyflexible droplet sensor with ultrahigh sensitivity and resolution, thedevice was applied to recording blood pressure variations on the skinsurface. In this dynamic measurement, a droplet sensor was devised witha built-in chamber size of 6 mm in radius and 140 μm in height,containing an electrolyte droplet of 3 μL as the sensing element toachieve a facilitated response (with the response time of 100 ms).

The sensor was attached to the skin surface above the carotid artery torecord blood pressure wave profiles with a gentle contact force beingapplied.

FIG. 8 depicts the minute blood pressure variations which were recordedwith the inventive pressure sensor having maximum pressure values lessthan 100 Pa at around 1 Hz. In view of the ultrahigh sensitivity andresolution, as well as having simple fabrication and flexibleconstruction; the inventive droplet-based capacitive pressure sensorshould prove attractive to a wide range of applications, while beingparticularly well-suited for biomedical applications (e.g., ocularsystems, pulmonary, and cardiovascular). It will be appreciated that inmost biomedical applications maximal pressure variations typically rangefrom 2 kPa to 20 kPa, while there is a need for devices that are bodyconformable and comfortable to allow for continuous pressure monitoring.In addition, the inventive sensors are substantially insensitive toevaporation when using the preferred electrolyte/glycerol mixture, andare substantially insensitive to thermal noises, which is a substantialbenefit over the use of resistive-based sensor schemes.

From the discussion above it will be appreciated that the invention canbe embodied in various ways, including the following:

1. A droplet-based capacitive sensor apparatus, comprising: a sensingchamber within an interior volume of a housing having a first and secondsubstrate between which are disposed a substrate separation structuremaintaining a periphery of said first and second substrates at a fixedseparation distance to form said sensing chamber; wherein at least oneof said first or second substrates is flexible; a conductive surfacecoating on the interior surfaces of said first substrate and said secondsubstrate to form a first electrode and a second electrode; and anelectrolyte droplet retained in said sensing chamber in contact withsaid first and second electrodes and leaving a gap between saidelectrolyte droplet and said substrate separation structure; whereincontact between said electrolyte droplet and said first and secondelectrodes form electric double layers (EDL) having interfacial EDLcapacitance proportional to interface contact area which varies inresponse to mechanical pressure applied to deform at least one of saidsubstrates which is flexible.

2. The apparatus of any previous embodiment, wherein a portion of saidfirst substrate, or said second substrate, or a combination of saidfirst and second substrate, centrally disposed in said housing, ismodified to be hydrophilic toward centrally anchoring said electrolytedroplet within said housing.

3. The apparatus of any previous embodiment, wherein a portion of saidfirst substrate, or said second substrate, or a combination of saidfirst and second substrate, disposed near the periphery of said housing,is modified to be hydrophobic toward enhancing central anchoring of saidelectrolyte droplet within said housing.

4. The apparatus of any previous embodiment, wherein a depression isformed in a central portion of said first substrate, or said secondsubstrate, or a combination of said first and second substrate, towardcentrally anchoring said electrolyte droplet within said housing.

5. The apparatus of any previous embodiment, wherein said electrolytedroplet contains at least 25% glycerol.

6. The apparatus of any previous embodiment, wherein at least a portionof said first and second substrates, having the conductive surfacecoating on said first and second electrodes, extends beyond said housingto provide electrical connection with said first and second electrodes.

7. The apparatus of any previous embodiment, wherein mobile electronsmigrate from said first and second electrodes and a counter-ion layeraccumulates from the electrolyte droplet in response to application of asensing voltage to said first and second electrodes.

8. The apparatus of any previous embodiment, wherein said housing andelectrolyte droplet are sized to leave a sufficient gap between saidelectrolyte droplet and said separation structure to allow deformationof the electrolyte droplet at maximum sense pressure without contactingsaid substrate separation structure.

9. The apparatus of any previous embodiment, wherein said firstsubstrate, or said second substrate, or both said first and secondsubstrates comprise a polymer membrane.

10. The apparatus of any previous embodiment, wherein said polymermembrane comprises polyethylene terephthalate (PET).

11. The apparatus of any previous embodiment, wherein said separationstructure comprises a polymeric material.

12. The apparatus of any previous embodiment, wherein said separationstructure comprises a polydimethylsiloxane (PDMS).

13. The apparatus of any previous embodiment, wherein said conductivesurface coating comprises indium tin oxide (ITO).

14. The apparatus of any previous embodiment, wherein said firstsubstrate to which is attached said first electrode, or said secondsubstrate to which is attached said second electrode, or a combinationof said first and second substrates and said first and secondelectrodes, comprise an optically transparent material.

15. The apparatus of any previous embodiment, wherein said separationstructure comprises an optically transparent material.

16. The apparatus of any previous embodiment, further comprising one ormore ventilation channels formed in said housing to maintain pneumaticpressure balance.

17. The apparatus of any previous embodiment, wherein said apparatus isconfigured for medical pressure sensing applications.

18. A droplet-based capacitive sensor apparatus, comprising: a sensingchamber within an interior volume of a housing having a first and secondsubstrate between which are disposed a substrate separation structuremaintaining a periphery of said first and second substrates at a fixedseparation distance to form said sensing chamber; wherein at least oneof said first or second substrates is flexible; a conductive surfacecoating on the interior surfaces of said first substrate and said secondsubstrate to form a first electrode and a second electrode; and anelectrolyte droplet retained in said sensing chamber in contact withsaid first and second electrodes and leaving a gap between saidelectrolyte droplet and said substrate separation structure; whereincontact between said electrolyte droplet and said first and secondelectrodes form electric double layers (EDL) having interfacial EDLcapacitance proportional to interface contact area which varies inresponse to mechanical pressure applied to deform at least one of saidsubstrates which is flexible; and wherein one or more portions of saidfirst substrate, or said second substrate, or a combination of saidfirst and second substrate, are modified to be hydrophilic towardcentrally anchoring said electrolyte droplet within said housing, or aremodified to be hydrophobic near the periphery of said housing towardpreventing said electrolyte droplet from moving into contact with saidhousing.

19. The apparatus of any previous embodiment, wherein a depression isformed in a central portion of said first substrate, or said secondsubstrate, or a combination of said first and second substrate, towardcentrally anchoring said electrolyte droplet within said housing.

20. The apparatus of any previous embodiment, wherein said electrolytedroplet contains at least 25% glycerol.

21. The apparatus of any previous embodiment, wherein at least a portionof said first and second substrates, having the conductive surfacecoating on said first and second electrodes, extends beyond said housingto provide electrical connection with said first and second electrodes.

22. The apparatus of any previous embodiment, wherein said housing andelectrolyte droplet are sized to leave a sufficient gap between saidelectrolyte droplet and said separation structure to allow deformationof the electrolyte droplet at maximum sense pressure without contactingsaid substrate separation structure.

23. The apparatus of any previous embodiment, wherein said firstsubstrate, or said second substrate, or both said first and secondsubstrates comprise a polymer membrane.

24. The apparatus of any previous embodiment, wherein said separationstructure comprises a polymeric material.

25. The apparatus of any previous embodiment, wherein said conductivesurface coating comprises indium tin oxide (ITO).

26. The apparatus of any previous embodiment, further comprising one ormore ventilation channels formed in said housing to maintain pneumaticpressure balance.

27. A droplet-based capacitive sensor apparatus, comprising: a sensingchamber within an interior volume of a housing having a first and secondsubstrate between which are disposed a substrate separation structuremaintaining a periphery of said first and second substrates at a fixedseparation distance to form said sensing chamber; wherein a depressionis formed in a central portion of said first substrate, or said secondsubstrate, or a combination of said first and second substrate, towardcentrally anchoring said electrolyte droplet within said housing;wherein at least one of said first or second substrates is flexible; aconductive surface coating on the interior surfaces of said firstsubstrate and said second substrate to form a first electrode and asecond electrode, which also extends beyond said housing to provideelectrical connection with said first and second electrodes; and anelectrolyte droplet retained in said sensing chamber in contact withsaid first and second electrodes and leaving a gap between saidelectrolyte droplet and said substrate separation structure; whereinmobile electrons migrate from said first and second electrodes and acounter-ion layer accumulates from the electrolyte droplet in responseto application of a sensing voltage to said first and second electrodes;and wherein contact between said electrolyte droplet and said first andsecond electrodes form electric double layers (EDL) having interfacialEDL capacitance proportional to interface contact area which varies inresponse to mechanical pressure applied to deform at least one of saidsubstrates which is flexible.

28. The apparatus of any previous embodiment, wherein one or moreportions of said first substrate, or said second substrate, or acombination of said first and second substrate, are modified to behydrophilic toward centrally anchoring said electrolyte droplet withinsaid housing, or are modified to be hydrophobic near the periphery ofsaid housing toward preventing said electrolyte droplet from moving intocontact with said housing.

29. The apparatus of any previous embodiment, wherein said electrolytedroplet contains at least 25% glycerol.

30. The apparatus of any previous embodiment, wherein said conductivesurface coating comprises indium tin oxide (ITO).

Although the description above contains many details, these should notbe construed as limiting the scope of the invention but as merelyproviding illustrations of some of the presently preferred embodimentsof this invention. Therefore, it will be appreciated that the scope ofthe present invention fully encompasses other embodiments which maybecome obvious to those skilled in the art, and that the scope of thepresent invention is accordingly to be limited by nothing other than theappended claims, in which reference to an element in the singular is notintended to mean “one and only one” unless explicitly so stated, butrather “one or more.” All structural, chemical, and functionalequivalents to the elements of the above-described preferred embodimentthat are known to those of ordinary skill in the art are expresslyincorporated herein by reference and are intended to be encompassed bythe present claims. Moreover, it is not necessary for a device or methodto address each and every problem sought to be solved by the presentinvention, for it to be encompassed by the present claims. Furthermore,no element, component, or method step in the present disclosure isintended to be dedicated to the public regardless of whether theelement, component, or method step is explicitly recited in the claims.No claim element herein is to be construed under the provisions of 35U.S.C. 112, sixth paragraph, unless the element is expressly recitedusing the phrase “means for.”

TABLE 1 Physical Properties of Electrolyte/Glycerol Mixture (A) (B) (C)Mix Con- c₀ (D) (E) (F) (G) Ratio ducts? (μF/cm²) Evap. ? ViscosityRelax. T Contact ∠ 100/0  Y 4.5 ± 0.14 Y 1.0 0.02 90.5 ± 0.71 75/25 Y4.7 ± 0.22 Y 2.5 0.06 89.5 ± 0.71 50/50 Y 4.4 ± 0.32 Y 8.4 0.18 ± 0.0286.0 ± 2.83 25/75 Y 4.4 ± 0.21 N 60.1 0.28 ± 0.02 89.5 ± 3.54  0/100 N —N 1412 2.29 ± 0.02 93.0 ± 1.41 (A) Mixing Ratio of theElectrolyte/Glycerol (v/v %) (B) Is it Electrically conductive? (C) Unitarea capacitance c₀ (μF/cm²) (D) Subject to Evaporation? (E) Viscosityμ(Pa · s) (F) Relaxation Time t(s) (G) Contact Angle (degrees)

What is claimed is:
 1. A droplet-based capacitive sensor apparatus,comprising: a sensing chamber within an interior volume of a housinghaving a first and second substrate between which are disposed asubstrate separation structure maintaining a periphery of said first andsecond substrates at a fixed separation distance to form said sensingchamber; wherein at least one of said first or second substrates isflexible; a conductive surface coating on the interior surfaces of saidfirst substrate and said second substrate to form a first electrode anda second electrode; and an electrolyte droplet retained in said sensingchamber in contact with said first and second electrodes and leaving agap between said electrolyte droplet and said substrate separationstructure; wherein contact between said electrolyte droplet and saidfirst and second electrodes form electric double layers (EDL) havinginterfacial EDL capacitance proportional to interface contact area whichvaries in response to mechanical pressure applied to deform at least oneof said substrates which is flexible.
 2. The apparatus recited in claim1, wherein a portion of said first substrate, or said second substrate,or a combination of said first and second substrate, centrally disposedin said housing, is modified to be hydrophilic toward centrallyanchoring said electrolyte droplet within said housing.
 3. The apparatusrecited in claim 1, wherein a portion of said first substrate, or saidsecond substrate, or a combination of said first and second substrate,disposed near the periphery of said housing, is modified to behydrophobic toward enhancing central anchoring of said electrolytedroplet within said housing.
 4. The apparatus recited in claim 1,wherein a depression is formed in a central portion of said firstsubstrate, or said second substrate, or a combination of said first andsecond substrate, toward centrally anchoring said electrolyte dropletwithin said housing.
 5. The apparatus recited in claim 1, wherein saidelectrolyte droplet contains at least 25% glycerol.
 6. The apparatusrecited in claim 1, wherein at least a portion of said first and secondsubstrates, having the conductive surface coating on said first andsecond electrodes, extends beyond said housing to provide electricalconnection with said first and second electrodes.
 7. The apparatusrecited in claim 1, wherein mobile electrons migrate from said first andsecond electrodes and a counter-ion layer accumulates from theelectrolyte droplet in response to application of a sensing voltage tosaid first and second electrodes.
 8. The apparatus recited in claim 1,wherein said housing and electrolyte droplet are sized to leave asufficient gap between said electrolyte droplet and said separationstructure to allow deformation of the electrolyte droplet at maximumsense pressure without contacting said substrate separation structure.9. The apparatus recited in claim 1, wherein said first substrate, orsaid second substrate, or both said first and second substrates comprisea polymer membrane.
 10. The apparatus recited in claim 9, wherein saidpolymer membrane comprises polyethylene terephthalate (PET).
 11. Theapparatus recited in claim 1, wherein said separation structurecomprises a polymeric material.
 12. The apparatus recited in claim 11,wherein said separation structure comprises a polydimethylsiloxane(PDMS).
 13. The apparatus recited in claim 1, wherein said conductivesurface coating comprises indium tin oxide (ITO).
 14. The apparatusrecited in claim 1, wherein said first substrate to which is attachedsaid first electrode, or said second substrate to which is attached saidsecond electrode, or a combination of said first and second substratesand said first and second electrodes, comprise an optically transparentmaterial.
 15. The apparatus recited in claim 1, wherein said separationstructure comprises an optically transparent material.
 16. The apparatusrecited in claim 1, further comprising one or more ventilation channelsformed in said housing to maintain pneumatic pressure balance.
 17. Theapparatus recited in claim 1, wherein said apparatus is configured formedical pressure sensing applications.
 18. A droplet-based capacitivesensor apparatus, comprising: a sensing chamber within an interiorvolume of a housing having a first and second substrate between whichare disposed a substrate separation structure maintaining a periphery ofsaid first and second substrates at a fixed separation distance to formsaid sensing chamber; wherein at least one of said first or secondsubstrates is flexible; a conductive surface coating on the interiorsurfaces of said first substrate and said second substrate to form afirst electrode and a second electrode; and an electrolyte dropletretained in said sensing chamber in contact with said first and secondelectrodes and leaving a gap between said electrolyte droplet and saidsubstrate separation structure; wherein contact between said electrolytedroplet and said first and second electrodes form electric double layers(EDL) having interfacial EDL capacitance proportional to interfacecontact area which varies in response to mechanical pressure applied todeform at least one of said substrates which is flexible; and whereinone or more portions of said first substrate, or said second substrate,or a combination of said first and second substrate, are modified to behydrophilic toward centrally anchoring said electrolyte droplet withinsaid housing, or are modified to be hydrophobic near the periphery ofsaid housing toward preventing said electrolyte droplet from moving intocontact with said housing.
 19. The apparatus recited in claim 18,wherein a depression is formed in a central portion of said firstsubstrate, or said second substrate, or a combination of said first andsecond substrate, toward centrally anchoring said electrolyte dropletwithin said housing.
 20. The apparatus recited in claim 18, wherein saidelectrolyte droplet contains at least 25% glycerol.
 21. The apparatusrecited in claim 18, wherein at least a portion of said first and secondsubstrates, having the conductive surface coating on said first andsecond electrodes, extends beyond said housing to provide electricalconnection with said first and second electrodes.
 22. The apparatusrecited in claim 18, wherein said housing and electrolyte droplet aresized to leave a sufficient gap between said electrolyte droplet andsaid separation structure to allow deformation of the electrolytedroplet at maximum sense pressure without contacting said substrateseparation structure.
 23. The apparatus recited in claim 18, whereinsaid first substrate, or said second substrate, or both said first andsecond substrates comprise a polymer membrane.
 24. The apparatus recitedin claim 18, wherein said separation structure comprises a polymericmaterial.
 25. The apparatus recited in claim 18, wherein said conductivesurface coating comprises indium tin oxide (ITO).
 26. The apparatusrecited in claim 18, further comprising one or more ventilation channelsformed in said housing to maintain pneumatic pressure balance.
 27. Adroplet-based capacitive sensor apparatus, comprising: a sensing chamberwithin an interior volume of a housing having a first and secondsubstrate between which are disposed a substrate separation structuremaintaining a periphery of said first and second substrates at a fixedseparation distance to form said sensing chamber; wherein a depressionis formed in a central portion of said first substrate, or said secondsubstrate, or a combination of said first and second substrate, towardcentrally anchoring said electrolyte droplet within said housing; andwherein at least one of said first or second substrates is flexible; aconductive surface coating on the interior surfaces of said firstsubstrate and said second substrate to form a first electrode and asecond electrode, which also extends beyond said housing to provideelectrical connection with said first and second electrodes; and anelectrolyte droplet retained in said sensing chamber in contact withsaid first and second electrodes and leaving a gap between saidelectrolyte droplet and said substrate separation structure; whereinmobile electrons migrate from said first and second electrodes and acounter-ion layer accumulates from the electrolyte droplet in responseto application of a sensing voltage to said first and second electrodes;and wherein contact between said electrolyte droplet and said first andsecond electrodes form electric double layers (EDL) having interfacialEDL capacitance proportional to interface contact area which varies inresponse to mechanical pressure applied to deform at least one of saidsubstrates which is flexible.
 28. The apparatus recited in claim 27,wherein one or more portions of said first substrate, or said secondsubstrate, or a combination of said first and second substrate, aremodified to be hydrophilic toward centrally anchoring said electrolytedroplet within said housing, or are modified to be hydrophobic near theperiphery of said housing toward preventing said electrolyte dropletfrom moving into contact with said housing.
 29. The apparatus recited inclaim 27, wherein said electrolyte droplet contains at least 25%glycerol.
 30. The apparatus recited in claim 27, wherein said conductivesurface coating comprises indium tin oxide (ITO).